<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    html,body{
  margin: 0;
  padding: 0;
}
/* 元素垂直水平居中 */
body{
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  background-color: black;
}
div{
  margin-right: 30px;
}

.moon1{
  width: 8em;
  height: 8em;
  border-radius: 50%;
  background-color: white;
  box-shadow: 0 0 20px white;
}


.moon2{
  position: relative;
  width: 8em;
  height: 8em;
  border-radius: 50%;
  background-color: white;
  box-shadow: 0 0 10px white;
}
/* 用伪元素::before 覆盖圆的一部分；
  也就是让其和父元素一样大小，背景黑色，相对定位覆盖圆的右边那部分 */
/* 缺少内弧的光辉 */
.moon2::before{
  content:'';
  width: 8em;
  height: 8em;
  border-radius: 50%;
  background-color: black;
  position: absolute;
  left: 2em;
  top: 0;
  z-index: 1;
}
</style>
<body>
    <div class="moon1"></div>
<div class="moon1"></div>
</body>
</html>